11"DD Coils

Is the 11" DD coil as fitted to my F75 SE useable on the Gold Bug Pro ??
Thanks Pyrites Pete

No........ different operating frequency.

Thanks Tom , Pyrites Pete

It's a good question and fair answer if one know what coils and detectors it concern.

However if one purchase a coil that looks right with regards to size, shape and plug type but have no other markings be it a number, letters or sign then the "What For" question arises real loud.

If someone can ID the coil in the picture or have a suggestion on how to, that would be greatly appreciated.

Suggestions of plugging it in to whatever detector the plug will fit and give it a go would not be my first choice the plug on this coil fits several models of Fisher Detectors,............and it also fits my Whites TDI, so it's not just a question of frequency.



ivanll

Plug it in if it works, fine, if not its for something else. A F75 coil will kind of work on the GB Pro but it probably won't ground bal.

In the picture, f-75 coil.

Looks like a early-model Teknetics T2 coil.

Measurement of resistance and possibly inductance on the appropriate pins could distinguish T2, F75 and 'other' models. Do you have a multimeter?

Looks like my Gold Bug DP coil.

How would you tell the difference?

Pimento Wrote:
-------------------------------------------------------
> Measurement of resistance and possibly inductance
> on the appropriate pins could distinguish T2, F75
> and 'other' models. Do you have a multimeter?

Coil 11x7"DD in pic. Resistance: Pin 1-2 = 57.4 ohm.....Pin 3-4 = 3.7 ohm

GB-Pro 5"DD Coil....Resistance: Pin 1-2 = 43.1 ohm.....Pin 3-4 = 3.1 ohm

The coil was purchased to be used on my GB-Pro, but seeing the answer that a 19 kHz coil will not work on a 13 kHz F75 SE made me wonder how one would see the difference and if I can's see it how would the person who sent it see it was for a GB.

Hope its not a matter of ............."this looks like it, that will do"

Hopefully the resistance measure will help with some answers.

In the meantime the coil is looking good just like any other 11x7"DD Fisher or Teknetics coil.

ivanll

Ivanll.
That's interesting. It's not a T2 coil (pin 5 is connected to 3,4 on that) and it doesn't match the resistance measurements of either of the F75's 5DD or 11x7DD coils (pin1-2=36 Ohms, pin3-4=2 Ohms). I would have expected the GBpro to have closely-matched 5 inch and 11x7 specifications, and your 11x7 has higher transmit-coil (3 Ohms) and receive-coil resistance (57 Ohms). If that eliminates the three most likely candidates, what is left? Without using 'scopes and L-meters, it's difficult to suggest any other tests.

The GB/G2 share the same coil as the Tek Greek series detectors and the F5 screw in type coil.

I've read compatability is likely between 19 kHz GB-Pro and Tek. Greek series coils for Alpha-Delta-Gamma-Omega.
So if someone have the Ohm readings on those coils it might shine a little light on what a coil without markings could perhaps be for.

Did a new Ohms testing on GB-Pro and F75SE coils with a FLUKE 83 Multimeter..........Looks like a different multimerer and perhaps weather temp can change the readings slightly.
Still as Pimento observed the wide gab in Ohms reading beteween 11x7" and 5" coils both supposed to be genuine and for a GB pro is kind of unexpected/strange.

GB-Pro 11x7"DD coil : Pin 1-2 = 55.6 Ohm......Pin 3-4 = 3.1 Ohm
GB-Pro.......5"DD coil : Pin 1-2 = 41.6 Ohm......Pin 3-4 = 2.5 Ohm

F75SE 11x7"DD coil : Pin 1-2 = 38.3 Ohm......Pin 3-4 = 2.3 Ohm
F75SE....... 5"DD coil : Pin 1-2 = 35.9 Ohm......Pin 3-4 = 1.8 Ohm

ivanll

I think one of the least critical parameters of a coil is the receive coil resistance, so engineering convenience might result in the difference between your two GB coils.
As well as having the correct inductance and resistance, the other critical parameter is the null (induction balance) of the coil. This is frequency-critical, and operating a coil at a different frequency to that intended will almost certainly result in a residual signal from the coil. If this is moderate, things will work OK, but the idea of Greek series (7.8KHz) coils and GB 19KHz coils being compatible/interchangeable is incorrect. Even if they had the same L and R values(unlikely,too).
That would be one of the laboratory tests you could perform - sweep the coil through the freq range, see where the best null is, distinguishing 7.8 / 13 / 19 Khz should be easy.

Pimento --

You stated that the idea of the Greek series coils and GB coils being compatible is "incorrect," in your post above. I'm not trying to be confrontational in any way, but I think you may be providing incorrect info...

Dave Johnson (the design engineer from Fisher) has stated that the Gold Bug Pro and G2 in fact ARE DESIGNED to work with the Tek "Greek Series" DD coils (not the concentrics that fit the Tek "Greek Series" machines). Here's a post he made in another forum (I hope it's OK for me to post this here...)

The new Bug (and also Tek G2) are designed to work with the Tek "Greek series" (also known as "Frat Bros." ) series DD searchcoils, which at this time (Nov 2010) consists of a 5 inch round and an 11 inch elliptical.

Some people have gotten creative and plugged in other searchcoils from the FratBros and F75 series just to find out what happens. Results have been mixed. Because of manufacturing variation in searchcoils some individual ones of a particular type may "work" and others may not work. The ones that "work" may in fact exhibit subtle performance deficiencies, or may seem to work fine today but not tomorrow.

FTP-Fisher explicitly un-recommends that people purchase anything other than searchcoils specified for use with these units. But if you have one of our other searchcoils anyway, there's nothing to stop you from plugging it in just to see what happens. It won't damage the machine. Note however that Tek T2 searchcoils flat out won't work, they're wired different.

This question crops up on such a regular basis that I invite the moderator to make it a "sticky".

--Dave J.

Steve

Very interesting, and surprising, too. You would have thought they would choose the existing coil nearest in frequency, ie. the 13Khz F75 unit.

Looking again at the recent Ivanll post, it seems the GB coil he has is very close to the 'mystery' coil in spec's. I assumed Ivanll had tried the coil and found it didn't work, maybe I'm wrong and he didn't want to risk it without doing other tests first. Correct, Ivan? I say go ahead and try it - no harm will come, for certain.

It's interesting what Dave J had to say about what can work 'sometimes'. Modern machines don't have conventional L-C transmit oscillators based around the search coil inductance, L and the detectors internal capacitor, C. They are force-driven at a quartz-crystal derived switching rate, and putting the wrong L (wrong coil) doesn't change the operating frequency, it just reduces the transmitted magnetic field (maybe by a lot).

On the subject of coil nulling, I have run an F75 coil at a range of frequencies (1KHz to 30KHz) and the null was definitely best at 13KHz, and even at 15KHz it was clearly 'off'. How tolerant of a poorly-nulled coil a detector would be is anyones guess. Hence my assumtion that a 7.8KHz coil would not be best at 19K.

pimento --

I was quite surprised, too...I don't know the electronic technicalities as you do, but having said that I also found it strange that Fisher would design the GB Pro/G2 to run with a coil that also comes stock on much lower-frequency units...

Steve

I slipped also Steve you are correct with the DD only coils listed for the GB.
Some have used concentrics and Dave J has said it will not hurt the detector but ID might the off.
Dave J also called tey Greek coils broad band and this is why the are used on the GB.

Garretts new AT Pro does use the same coil of the AT Pro Gold running different Freq one at 15 and the other at 19.
The CZ , DFX and M/L have been runnning two or more Freqs in one coil for some time now.

Been looking for the new Fisher 5x10"DD, it to seem to have some "broad band" uses. ( Tek. greek series, G2...Fisher F5 and GB + Pro) if I manage to get one the pin resistance will be checked.

The mystery coil was obtained for the GB Pro at the time of purchase and it do work in that combination, but it will also work on my F75SE with a reduced sensitivity setting.

Had water leaking from the groove at the cover a week after using it at the beach (always stick the coil under a running fresh water tap after use) so I popped the unsealed cover to see why and check it out.

This what it looks like.......... after drying out.


ivanll

I do not see much corrosion. Does it still function per design?

Give the small circuit board a brush to remove any salt crystals/debris. Everything else looks sealed up. If it still works OK on the GB, seal it up. If you use silicone sealant, I would advise using 'Low-Odor' types, which don't give off acetic acid (vinegar) fumes when curing. These could potentially cause corrosion on the circuit board.
If you're curious, the small loop of wire is the balancing coil, used to fine-tune the null, by moving it's position around in the cavity.

My apology for returning to this post and for the link to a dealer, it is for a good purpose.
[www.relichuntersupply.com]

I did clean and assemble the unnamed 11x7"DD coil and it is working, if it is per design......only the maker will know.


Got the 10x5"DD and tested the resistance to see.
Pin 1-2 = 58.8 Ohm. Pin 3-4 = 3.4 Ohm. Not far of the Greek series coil...........That was the reason for the above link, shows the Greek named detectors.

Also tested the NEL Tornado coil, said to be made for the Gold Bug.
Pin 1-2 = 53.5 Ohm. Pin 3-4 = 3.2 Ohm.

This should conclude the "what is it question" I will accept it might be a Greek series without a sticker and just uset for all it's worth.

Thank's all for your participation and guidance.

ivanll